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Successful storage of potatoes

Can you pass the storage fitness test?


November 14, 2007
By Dr. Robert Coffin and William Hardy

Many growers think that once the potato crop has been harvested, 90 percent
of the crop management has been completed. Nothing could be farther from the
truth.

Potato tubers are 'living' organisms and have specific needs to maintain high
quality during storage. Whether the potatoes are destined for table, seed or
processing markets, optimum storage management will pay dividends in assuring
the parameters of desirable quality are met.

Each year, millions of pounds of potatoes rot in storage and a considerable
amount of this rot could be prevented with improved management programs.

Tuber biology
Potato plants with green leaves are 'factories' that capture energy from the
sunlight and produce sugar. The sugar (sucrose) is moved from the leaves to
the developing potatoes (tubers) where it is changed to starch as the tuber
matures.

In order to keep the crop healthy during the growing season, farmers have numerous
challenges to control and suppress a wide range of insect and disease pests.
Infection of tubers in the field by pink rot and/or late blight fungi can pre-dispose
the tubers to storage rot. Damage to the potato foliage by pests can result
in reduced yields of potatoes.

When mature, an average tuber is 80 percent water and 20 percent dry matter.
Potato tubers are nutritious, they contain carbohydrates, vitamins, minerals
and protein, and a wide array of fungi and bacteria will grow rapidly on improperly
stored tubers, leading to storage rot and significant financial losses to the
farmers.

Potatoes for seed and table use are stored at three to four degrees C and processing
potatoes for chips and fries are stored at nine to 10 degrees C. Storage of
processing potatoes at colder temperatures leads to sweetening as starch is
converted to sugar, and this results in dark fries and chips. Microbial activity
is higher at the higher storage temperatures.

Grower education
Cavendish Farms conducts numerous trials to test new varieties, irrigation practices,
fertilizer rates and pesticides. The results are shared with growers by hosting
field tours of the plots, which are followed by a presentation of crop yield
and quality data. An added component of the company's potato program is the
provision of an extension service to growers for crop management in the field
and storage. One of the most important parts of that service is in storage management.

Information is delivered to growers in two ways: through storage workshops,
and through individual farm visits where a 'storage fitness test' is completed
and recommendations are made.

Three components are evaluated during the storage visit: the growers' work
habits and knowledge base of physics and plant physiology, the condition of
the potato crop and, the condition and design of the storage building.

Based on the experiences observed and the information gathered through numerous
farm visits, Cavendish Farms' potato extension experts have learned what the
major challenges are and how to address the issues.

During storage workshop sessions, instructors stress the importance of understanding
that potatoes are 'living' organisms and just like humans, they respire and
generate considerable amounts of heat, carbon dioxide and water.

Cavendish Farms held its first grower workshop six years ago during the winter
months. Prior to the arrival of the growers, the windows were opened to cool
and refresh the air in the room. The windows were closed before the meeting
commenced. Meters were hidden in the room to measure the air temperature, carbon
dioxide concentrations and relative humidity (RH). Every half hour, the instructor
discretely looked at the meters and wrote the numbers on the black board. Finally,
one grower asked what the numbers meant, presenting an excellent opportunity
to explain that the carbon dioxide levels had increased from 500ppm at the start
of the meeting to 2500ppm in two hours and the temperature had moved from 16
degrees C to 22 degrees C. The relative humidity had not changed but some growers
doubted the fact that there was more water vapour in the air after two hours
(warm air has greater water holding capacity).

The instructor told the growers in the enclosed meeting room that they were
in a similar environment to a storage room full of potatoes without air exchange
and the 'message' now had some relevance!

Storage fitness
In order to facilitate storage inspections and to review observations with the
grower, Cavendish Farms developed a checklist that is referred to as a 'Storage
Fitness Test'. Some of the key points on the list to check for are:

  • balanced air flow through all of the pile,
  • temperature gradients in the pile,
  • deteriorating insulation,
  • air leakage around doors,
  • accurate assessment of RH,
  • 'free' moisture on potatoes,
  • excessive carbon dioxide concentrations,
  • fruit flies,
  • ammonia odour, and
  • accuracy/precision of control panel and monitoring equipment.

In some instances, the storages visited scored well while others had some glaring
problems that once addressed, led to improved success in preventing storage
rot. Although most growers have excellent storage monitoring equipment, the
'green light' outside of the storage can provide a false security: it does not
guarantee that all is well within the storage.

The most successful growers personally visit their potato storages every day
and use their eyes and nose for early detection of problems. If a grower walks
into potato storage and observes fruit flies and smells ammonia, they know there
is a problem 'brewing' somewhere in the building and immediate attention is
required. There are times the storage operator needs to make a judgment to over-ride
the computer controlled panel to provide the best environment for the potatoes.

One of the most critical aspects to assure successful storage of potatoes is
the maintenance of an optimum relative humidity. If the RH is too low, potato
tubers will lose excessive moisture and shrivel. If the relative humidity is
too high, the circulating air will not dry up any 'leaking' potatoes. Some growers
try to 'guess' the relative humidity by the amount of condensation on the ceiling,
but this can be misleading if there is insufficient insulation. The best way
to accurately determine the relative humidity is with a wet bulb-dry bulb psychrometer.
Users should make sure that the two thermometers are reading the same when the
machine is 'dry' and not in use. Improperly paired thermometers can lead to
inaccurate calculations of RH. If there is no rot in the pile, it is acceptable
to maintain the RH at 92. If there is less than two percent rot, the operator
should try to adjust the RH to 85 so the circulating air has 'drying' capacity.
If there is more than two percent rot, the grower should not store the potatoes.

Extension personnel have encountered a wide range of situations during storage
visits. Some problems were easy to correct while others were quite a challenge.
On a simple issue, a grower complained that the computer would not permit air
intake during some relatively cold days. Upon inspection, it was observed that
the outside temperature sensor was located on the sunny side of the building
in a sheltered place. It was giving a false high temperature reading, hence
the system would not permit air intake if it was deemed to be too high for cooling.

On a more serious visit, personnel entered a storage with 'free water' on the
potatoes, floor and walls. Fruit flies were numerous, an ammonia odour was present,
carbon dioxide readings were in the 6000ppm range rather than the normal 1200
to 1500ppm and the RH was 99! Immediate intervention was implemented in an effort
to save the crop. Air was leaking around the front door, permitting cold air
to enter the building and upon mixing with the warmer humid air; it condensed
and released considerable free water. Once the air leaks were corrected and
a portable dehumidification system was activated, the storage floors and potatoes
became dry within 48 hours. The leaking potatoes started to dry-up and potential
for the spread of rot was reduced. A pending disaster was narrowly averted.

The introduction of variable speed drives for air flow has reduced the electricity
costs for ventilation. Although savings in electricity are enticing, making
changes in air circulation patterns must be done carefully to avoid additional
problems. -30-

*Dr. Robert Coffin, Ph.D., P.Ag.,
is a crop specialist with Cavendish Farms and an adjunct professor at UPEI and
NSAC. William Hardy, Dip. Agr., is a research assistant with Cavendish Farms.
They are both based in Prince Edward Island. Coffin recently spoke on storage
management during the World Potato Congress, held in Boise, Idaho during August.
He enclosed some of the key points from his presentation in this article.

 


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